Polyethylene is a crystalline, non-polar thermoplastic resin. Polyethylene is also easier to mold and has higher strength and elongation, flexibility and chemical resistance than other thermoplastic or thermosetting resins. Thus, polyethylene has excellent properties and is one of the most important general-purpose resins. However, polyethylene is disadvantageous in that its lack of polar groups causes poor adhesion, oil resistance, dyeability, and heat resistance.
Various studies have been made to improve polyethylene so that it is given characteristic properties without deteriorating the above inherent properties thereof to be developed for formation of composite materials with other materials.
In particular, copolymerization of ethylene and a vinyl compound having polar groups is widely industrially conducted because it is advantageous in that it enables the introduction of polar groups in a high concentration and the production thereof in the same plant as used.
However, high-pressure radical copolymerization of ethylene and such a comonomer having polar groups involves a problem of reactivity of the comonomer. That is, comonomers having a relatively small polymerization reactivity (i.e., the Q value is less than 0.1) such as vinyl acetate and vinyl propionate are generally introduced into a system at a line having a pressure of lower than the reaction pressure, such as, for example, at an inlet line of an ultrahigh pressure compressor, compressed together with ethylene by the ultrahigh pressure compressor to the reaction pressure, and then introduced into the reaction vessel. On the other hand, in the case where comonomers having a high reactivity are used, when a mixture of ethylene and such a comonomer is compressed by an ultrahigh pressure compressor, a copolymer of ethylene and the comonomers or a homopolymer of the comonomer may be produced in an interior of the compressor, an outlet conduit of the compressor, and other apparatus due to compression heat. The polymer thus produced attaches to the interior of the compressor, the outlet conduit of the compressor and other apparatus, causing not only an increase in the amount of leaking gas from a cylinder of the compressor and an increase in pressure loss between the compressor outlet and the reaction vessel which will increase the discharge pressure of the compressor but also a cause to clog the conduits, whereby it causes a very dangerous condition in operation. In order to avoid these troubles, when comonomers having a high reactivity are used, the comonomers are compressed to the reaction pressure by an ultrahigh pressure pump, singly or together with a solvent, and then carried to the reaction vessel.
In general, as such comonomers there are used those which are liquid or solid at ordinary temperature and at atmospheric pressure. A plunger type pump is used as the ultrahigh pressure pump to compress the comonomer to the reaction pressure and carry it to the reaction vessel. As a packing for this type of a pump there is generally used a self-bound V-shaped packing. However, when a comonomer having a high polymerization reactivity enters and accumulates in a V-shaped groove on the packing, heat polymerization takes place in a relatively short period of time due to sliding heat generated in sliding portions of the plunger and the packing. This causes a loss of degree of freedom of the packing, resulting in leakage therefrom. Furthermore, the attachment of the polymer thus produced may even lead to breakage of the pump.
Moreover, the polymer thus produced also accumulates in the vicinity of inlet and discharge valves and in the conduits of the pump, causing an increase in loss of the pressure in conduit which may cause clogging of the conduit. This thus causes a very dangerous condition in operation.
As the method for carrying the comonomers to the reaction vessel, there is generally employed a method in which the comonomers are carried directly to the reaction vessel through a single conduit provided between the ultrahigh pressure pump for compression of the comonomers and the reaction vessel. In this process, however, there is a high possibility that by heat transfer from the wall of the reaction vessel the temperature in the vicinity of the inlet nozzle attached to the wall of the reaction vessel rises close to the reaction temperature (generally 150.degree. to 250.degree. C.), to cause heat homopolymerization of the comonomer. In particular, when the reaction is interrupted or stopped, since the comonomers remain in the vicinity of the nozzle, there is a high possibility that polymerization occurs therein. When a polymer produced by heat polymerization of the comonomer in the vicinity of the nozzle attaches to the wall of the conduit, the pressure loss between the pump and the reaction vessel increases, which causes an increase in the discharge pressure of the pump, whereby the operation is in a dangerous state. Furthermore, when the comonomers undergo heat polymerization in the vicinity of the nozzle after the operation is finished, it causes clogging of the nozzle which may break the pump when the operation resumes. In order to avoid these troubles, when the reaction is in progress, the reaction must be immediately interrupted, or when the reaction is finished, the pump, the conduits and the inlet nozzle must be immediately dismantled and cleaned.
Moreover, when a comonomer having a high homopolymerizability, i.e., a comonomer having a Q value, as defined hereinafter, of 0.1 or more, is directly introduced into the reaction vessel, such a comonomer is not sufficiently dispersed into ethylene gas in the reaction vessel. This causes local homopolymerization of the comonomer by-production of gel-like materials in the finished product or likely leads to abnormal reaction. Furthermore, the resulting copolymer often lacks randomness, thus giving a trouble in the properties of the product.
Thus, the production of an ethylene copolymer using a comonomer having a high reactivity has difficulty in prolonged operation and is remarkably poor in operation efficiency and safety as compared with the production of polyethylene and the production of a copolymer of ethylene and a comonomer having a low reactivity.